1. Field of the Invention
The present invention relates to a method for treating a zinciferous metal surface, i.e., one with a surface composition that is at least 45% zinc by weight. The method is particularly suitable for treating zinc-plated steel sheet and will be generally so described herein below, but treatment of other zinciferous surfaces is within the broadest scope of the invention. More specifically, the present invention relates to a treatment of the zinciferous surface before applying a conventional chromating treatment; the pretreatment of this invention increases the resistance of the later applied chromate film to the development of black and/or white types of rust on the zinciferous surface. The invention is particularly suited to treating electrogalvanized steel sheet and hot-dip galvanized steel sheet.
2. Statement of Related Art
The sacrificial corrosion protection of iron and steel using zinc plating has proven to be the most effective and economical, with the result that 10% of Japan's annual raw steel output of 100 million tons--or 10 million tons--is produced as zinc-plated steel sheet. This zinc-plated steel sheet is used in a number of fields, e.g., as a building material, for automobiles, and in household electrical appliances.
In sacrificial corrosion protection by zinc, the two metals, zinc and iron (or steel), form a cell when brought into contact. As the baser metal, zinc becomes the anode, thus cathodizing the iron and suppressing the anodic dissolution caused by the formation of local cells in the case of iron alone. This results in an inhibition of the corrosion of the iron or steel. Accordingly, the anticorrosive action ceases when the zinc in contact with the iron or steel has been consumed, and this fact requires that corrosion of the zinc layer itself be inhibited if it is to exercise long-term activity. Chromating is conducted after plating as a countermeasure to zinc corrosion.
However, chromating-based rust prevention suffers from the following deficiency: While the chromating of zinc plated steel sheet does result in a substantial inhibition of white rust development, the development of black rust (known as blackening) still occurs during storage and transport. This black rust impairs the external appearance. It has been observed that this blackening phenomenon occurs more readily for steel sheet subjected to a post galvanization skin pass and for steel sheet plated with a Zn/Al alloy containing several % Al than in the case of ordinary zinc-plated steel sheet.
Flash treatment, by Ni.sup.2+ and/or Co.sup.2+ containing aqueous solution for example, as described in Japanese Patent Application Laid Open [Kokai] Number 59-177381 [177,381/1984], is currently regarded as a powerful countermeasure for solving this post-chromating black rust problem. (Here and below, flash treatment is taken to mean the chemical deposition of very small quantities of metal.)
In its essential features, the above-referenced invention concerns a means for inhibiting black rust through the pre-chromating flash treatment of the surface of zinc plated or zinc alloy plated steel sheet. In this particular flash treatment, the surface of the steel sheet is flash treated with an aqueous solution (pH 1 to 4 or 11 to 13.5) that contains the Ni.sup.2+ and/or Co.sup.2+ metal ion(s). This treatment results in the deposition of these metal ions on the plating surface as the metal or oxide. This treatment is followed by a water wash and then by the formation of a chromate film.
The mechanism has yet to be clearly established for black rust inhibition on zinc-plated or zinc alloy-plated steel sheet that has been Ni and/or Co flash-treated and subsequently chromated. However, according to the report on pages 150 to 151 of Abstracts of the 60th Scientific Lecture Conference of Kinzoku Hyomen Gijutsu Kyokai [Metal Finishing Society of Japan], the flashed metal is largely deposited at the zinc crystal grain boundaries. It is further reported that the chromium compounds laid down in a follow-on application-type chromating treatment are, like the flashed metal, also distributed at the grain boundaries. Presumably there is some type of interaction between the flashed metal and the chromium compounds, and the chromium compounds are thereby adsorbed and fixed on the flashed metal.
Zinc black rust, like the corresponding white rust, is thought to be a basic zinc carbonate (ZnCO.sub.3).sub.x.[Zn(OH).sub.2 ].sub.y ; however, it differs from white rust in that it is oxygen deficient on a stoichiometric basis. Black rust is therefore a corrosion product occurring under oxygen-deficient conditions, and in particular it is thought that black rust is formed in association with the development of corrosion from the grain boundaries. As a result, it can be postulated that the chromium compounds concentrated at the grain boundaries by the flashed metal contribute to an inhibition of black rust development by inhibiting corrosion originating from the grain boundaries.
As stated above, the Ni and/or Co flash-treatment of zinc-plated steel sheet prior to its chromating is in fact a powerful countermeasure against the problem of black rust. However, this flash treatment prior to chromating is deficient in that it permits the rather facile development of white rust despite its inhibition of black rust.
One pre-chromating treatment that improves the blackening resistance, corrosion resistance, post-painting corrosion resistance, and paint adherence and that does so while maintaining a good balance among these properties is disclosed in Japanese Patent Application Number Hei 2-275 837 [275,837/1990], filed 15 Oct. 1990. In this method, an aqueous solution of one or more selections from the sulfate and phosphate salts of Ni.sup.2+ and Co.sup.2+ is applied to the zinc or zinc alloy surface prior to chromating. Drying then affords a film at a loading of 0.5 to 100 mg/m.sup.2. While this method does achieve its intended effects, zinc elutes from the plating surface due to reaction with the sulfate in the treatment bath. As a result, when the treatment bath is recycled, a zinc reaction precipitate gradually accumulates in the bath. This makes the bath unstable, that is, its performance gradually deteriorates.
One prior-art pre-treatment method consists of a water wash subsequent to the substitutional plating of Ni.sup.2+ and Co.sup.2+ in an alkaline bath that contains a complexing agent such as ammonia, organic acid, and the like. However, in this pre-treatment method, almost all of the Ni and Co retained on the treated surface is believed to be precipitated as metal. Thus, while this pre-treatment method does in fact improve the black rust resistance and paint adherence, local cells can be formed with the Zn and this results in a strong microcorrosion tendency and a deterioration in resistance to white rusting.